The present invention relates to an improved toner for electrophotography, which is suitable for high speed development of latent electrostatic images and fixing the developed images, comprising a particular resin having excellent anti-offset and anti-blocking properties.
Recently electrophotography is applied not only to copying machines, but also to laser printers. In accordance with this trend of the application of electrophotography, there is a keen demand for high speed recording by electrophotography. Thus, heat rollers suitable for high speed image fixing are widely used as image fixing unit for electrophotogaphy. Furthermore, since developed or printed images on copy sheets are apt to be bent when used in practice, for instance, in direct mail, and post cards, improved image fixing performance is required for printing by electrophotography.
In Japanese Laid-Open Patent Application 50-134652, there is proposed a toner for electrophotography comprising as a binder agent therefor a styrene - acryl copolymer resin having a degree of dispersion (Mw/Mn) of 3.5 to 40 as being excellent in anti-offset property, where Mw represents the weight average molecular weight of the copolymer and Mn represents the number average molecular weight of the copolymer.
Furthermore, in Japanese Laid-Open Patent Application 62-115170, there is proposed a toner for electrophotography as being suitable for high speed image fixing, and as the anti-offset and anti-blocking properties being improved, which comprises a styrene - acryl copolymer resin (A) having a weight average molecular weight of 10,000 to 30,000 and a glass transition temperature of 60.degree. C. to 70.degree. C., and a styrene - acryl copolymer (B) having a weight average molecular weight of 200,000 to 400,000 and a glass transition temperature of 55.degree. C. to 65.degree. C., with a mixing ratio by weight of the copolymer (A)/copolymer (B) being in the range of 90/10 to 50/50, and has a melt viscosity of 10,000 poise or less at 140.degree. C.
However, the above conventional toners have the drawbacks that in the case where the toners are used as liquid developers, they cannot be dispersed in a stable manner in aliphatic hydrocarbon solvents, and in the case where the toners are used as dry toners, the image fixing performance is insufficient for use in practice when applied image fixing energy is low.